This invention relates to a process for selectively desulfurizing gases produced by reacting fuels with oxygen containing gases and water vapor under pressure. More particularly, this invention relates to a process for desulfurizing gases utilizing an alkali salt concentrated scrubbing solution at a certain temperature while maintaining a certain exchange of volume ratio.
Fuel gases which are produced by a gasification of solid or liquid fuels with oxygen, air or mixtures thereof and/or with steam and which are subsequently purified, particularly to remove sulfur compounds and, if desired, carbon dioxide, must be cooled before such purification from the high temperature of the gasifying reaction, which exceeds 500.degree.C., to a temperature which is suitable for the purification and is below 150.degree.C., preferably to the ambient temperature or below the same, e.g., down to -70.degree.C. This cooling may result in a removal of certain components of the gas and such removal may constitute a loss as far as the subsequent stages are concerned in which the gas is utilized. Such components are, e.g., water vapor, which could be used for a shift conversion of carbon monoxide to form carbon dioxide and hydrogen, or hydrocarbons which are still vaporous at higher temperatures and which would contribute to the caloric value of the purified gas, particularly if the gas which has been produced is utilized as a fuel gas in a gas turbine. A gas for this purpose should be combustible, free of sulfur and as hot as possible. The value of the gas depends more on its volume than on its caloric value. For this reason, the fuel which is available is preferably gasified with air. Such gas may contain water vapor and carbon dioxide as components which are useful because they increase the volume.
Known processes of desulfurizing hot gases are compositions which absorb sulfur compounds and which consist, e.g., of iron oxide or zinc oxide and combine with sulfur to form sulfide. Their use is expensive and requires a gas which has been prepurified to a large extent and particularly is free of water vapor. For this reason, these processes are mainly used for a fine purification of pretreated gases or as a safety measure.
Gases are conventionally purified in scrubbing processes in which the substances to be removed from the gas are taken up by physically or chemically acting absorbent solutions.
In physically acting absorbent solutions, the gas components are dissolved in dependence on their absorption coefficient and their partial pressure. In accordance therewith, the aborption capacity of the physically acting scrubbing solution increases with increasing pressure and decreasing temperature. For this reason, scrubbing processes using physically acting absorbents are carried out under elevated pressures above about 10 kilograms per square centimeter and with high-boiling organic solvents, such as ethylene glycol, diglycol ether, propylene carbonate, N-methylpyrrolidone, at ambient temperature or with low-boiling organic solvents, particularly methanol, at temperatures below -10.degree.C. down to about 70.degree.C.
In chemically acting absorbent solutions, several gas components, particularly those having an acid function, specifically sulfur compounds and carbon dioxide, are chemically combined. Chemical absorbent solutions are generally required to lend themselves to easy regeneration, i.e., to a reversal of the absorption reaction under simple conditions.
Scrubbing processes using chemically acting absorbents are less dependent on pressure. They may be carried out under ambient pressure and at ambient temperature. The laden absorbent solution may be regenerated by boiling and stripping with steam or with the aid of air.
Suitable absorbent solutions are aqueous solutions of strong organic bases or of alkali salts of inorganic or organic acids. The aqueous solutions of weak acids may be used also at higher absorption temperatures, although an elevated pressure must then be applied. In this case, the regeneration is effected by a pressure relief to a lower pressure, preferably to ambient pressure, with boiling and stripping with steam.
U.S. Pat. No. 2,886,405 discloses a process of scrubbing fuel gases and synthesis gases to remove acid components, particularly carbon dioxide. In that process, the absorption is effected in a hot concentrated solution of potassium carbonate at a temperature near the atmospheric-pressure boiling point of the solution, and the laden solution is regenerated by a pressure relief and by boiling and stripping with the steam which is produced in the boiling solution or which is additionally introduced.
That known scrubbing process is particularly suitable for a removal of the large amounts of carbon dioxide which have been formed by shift conversion of carbon monoxide with water vapor to form carbon dioxide and hydrogen in the production of synthesis gases or hydrogen.
That process know as the hot potash scrubbing process may be used to remove sulfur compounds, particularly H.sub.2 S together with the carbon dioxide, from the gas to be purified. So far, it has not been possible to accomplish a selective absorption of the sulfur compounds before the absorption of carbon dioxide under the conditions of the hot potash scrubbing process.